Due to increases in medical knowledge, and significant advancements in surgical techniques, the design and material selections for knee prosthetic joint implants and procedures for total knee arthroplasty (TKA) are changing. The increase in middle-aged and elderly persons occasioned by the “baby boom”, coupled with the desire of those persons to maintain an active lifestyle has led not only to the procedures becoming more frequent but for increased demand for more normal function of the artificial joints.
The design of knee prosthetic implants has seen considerable change since its inception based primarily on a better understanding of the motion and stability of the natural knee. Initially it was thought that the articulation of a human knee could be adequately mimicked by a prosthetic implant designed as a mechanical hinge that allowed flexion-extension rotation about a single axis. Such devices suffered significant limitations, both in the range of motion of the patient's knee joint but also in breakage of the mechanism. Analysis of human knees in living patients using MRI analysis and the study of cadaver knee joints have added significantly to the understanding of the kinematic and stability behavior of the living knee. Most significant is the understanding that during the majority of flexion, the medial compartment of the knee (medial femoral condyle, medial meniscus and tibial plateau) moves largely as a simple ball-in-socket joint, whereas the lateral compartment (lateral femoral condyle, lateral meniscus and lateral tibial plateau) moves such that the contact point changes from anterior to posterior (front to back) in an arcuate path dependent largely on the longitudinal rotation of the tibia beneath the femur. Thus the contact area or point (imaginary point at the center of contact pressure) between the medial condyle of the femur and the medial tibia remains almost stationary while the lateral condyle contact point on the lateral tibia moves during activity. The behavior of the knee in the terminal 20 degrees of extension and after 115 degrees of flexion differs from the motion in the mid-range of motion. In full extension, a cam-like effect of the distal medial articular surface moves the femur away from the tibial surface, tightens the medial and lateral collateral ligaments, and creates an external rotation of the tibia called “screw-home”. Beyond 115 degrees of flexion the lateral tibial contacts the posterior lateral tibial surface in a rolling and sliding motion that moves the lateral femoral condyle off the back of the tibia (“roll off”) and allows full flexion while decompressing the contents of the popliteal fossa. Thus the knee has three ranges of motion: full-extension, midrange flexion, and full-flexion.
Stability at the medial side of the knee is provided by the conformity between the femoral condyle and the concave tibial surface with the attached medial meniscus such that the construct resists anterior-posterior (AP) displacement, while the lateral side of the knee with its convex surface and less firmly attached meniscus does not provide a high degree of AP constraint but is free to move in and anterior-posterior manner with knee flexion. This kinematic behavior provides that at the medial interface acts as the center or rotation for internal/external rotation of the tibia while the lateral femoral condyle moves anteriorly and posteriorly. An elongated distal contour of the medial femoral condyle creates the cam-effect and screw-home motion at the last 10-15° of extension. The anterior-posterior mobility of the lateral compartment allows the unique motion in flexion beyond 115 degrees.
Knee prosthetic implants ideally seek to match the kinematic and stability behavior of a healthy living knee. Paramount is maintaining stability of the joint through the full range of motion. Currently available knee prosthetic implants rely heavily on tension in the medial and lateral collateral ligaments with or without tension in the posterior cruciate ligament to drive kinematics and stability. It is conventionally understood that adjusting (through “ligament balancing”) and maintaining tension in these ligaments throughout flexion is essential to providing stability of the joint. However, in the total knee arthroplasty procedure it is difficult to achieve desired tension in these ligaments and lack of the desired tension or excessive tension leads to a result unacceptable to the patient. Insufficient ligament tension leads to joint instability and a result unacceptable in terms of function to the patient, and in extreme cases dislocation of the joint. Excessive ligament tension leads to restrictions in range of motion and the potential for ligament failure. Moreover, most designs fail to provide a full range of flexion movement because the type of stability and motion found in the normal knee (referenced above) is not provided by the articulation and ligamentous tension.